1. Field of the Invention
This invention relates in general to noise reduction apparatus for reducing the detection errors caused by noise impulses in communications channels, and especially to such noise reduction apparatus employing memory and nonlinear techniques to examine the signal waveform prior to a noise impulse, remove the impulse, and substitute for the impulse the most likely waveform trajectory based on the signal's trajectory prior to the noise impulse.
2. Description of the Prior Art
Communications channels are subject to two types of noise. The first type is thermal or Gaussian noise caused by the random motion of charged particles (usually electrons) in the channel. Gaussian noise is characterized by wide bandwidth and relatively low magnitude. The second type of noise, noise impulses, are often produced by man-made devices, including engine ignition systems and motors. The impulses are characterized by wide bandwidth, fast rise and fall times, short duration compared to the highest frequency signal element, and possibly large amplitude. Traditional communications receiver design neglects impulsive noise and considers only Gaussian noise in the communications frequency band. To reduce the effects of in-band Gaussian noise, receivers employ channel selection filters to eliminate out-of-band Gaussian noise or sufficient signal power must be available to yield a useable signal-to-noise ratio at the receiver (considering channel attenuation and in-band Gaussian noise power at the receiver).
The impulses, which are often orders of magnitude larger than the peak information signal amplitude, can totally obliterate communications. If, however, they are of short duration compared to a transmitted digit interval or a cycle of the highest analog-signal frequency, their effect can be totally eliminated using prior art nonlinear schemes discussed below. Even longer impulses disrupting one or more message elements can be prevented from interacting with receiver filters by these prior art techniques, thereby avoiding prolonged disruption of the signal; an error-correcting code can be used to achieve reliable message transmission despite impulse-induced detection errors on some data bits or signal portions. Since a large class of real-world interference sources are of the impulse type, their effect can be diminished using conventional prior art nonlinear techniques, and especially the memory-nonlinear techniques of the present invention.
The prior art nonlinear techniques and the memory-nonlinear technique of the present invention are discussed herein in terms of digital signals and periodic sampling of signal waveforms. These discussions are also valid for continuous-time processing of digital signals and for analog signals. The techniques are applicable to any signal type, either baseband or modulated-carrier.
A communications receiver usually includes frequency-selective filters to reject unwanted signals and out-of-band Gaussian noise. When shocked by a large-amplitude broadband impulse, these filters produce a prolonged output, longer than the original impulse, that may destroy a long string of received data bits. For any type of nonlinear circuit to be useful, it must differentiate between the time-domain properties of the impulse and the signal. By cutting off or blocking out signal voltages rising too fast or too high to have been transmitted as part of the desired message, the prior art nonlinear circuits actually remove impulse energy lying both inside and outside the communications frequency band. The in-band portion would otherwise pass through the filter circuits and produce the prolonged output. A nonlinear circuit, placed ahead of the filters, takes advantage of the distinct time-domain properties of the impulse to reflect most of the energy before it enters the filter circuits. To preserve the shape of the impulse before it reaches the nonlinear stage, the channel portion through which the impulse propagates, and any receiver circuits ahead of the nonlinear circuit, must have a relatively wide bandwidth. In complex receivers with several stages of increasingly fine sub-channel selection, a single nonlinear circuit can be placed ahead of the first filter stage, or several nonlinear circuits can be used ahead of successively more selective filter stages. Several prior art examples of such nonlinear circuits are discussed below, including clippers, noise blankers, and hard limiters.
These prior art nonlinear circuits employ zero-memory nonlinearity techniques. That is, the output at any instant depends on the input only at that same instant. Masking of the noise impulse can be greatly improved, however, if the nonlinear circuit considers the immediately previous history of the signal and attempts to insert an estimate, using this previous history, for that portion of the signal corrupted by the impulse. The signal remains a smooth wave, free of clipped spikes, possibly of the wrong sign, or holes in the signal where it was corrupted by the noise impulse. The insertion of an accurate estimate during the time of corruption, based on the previous state of the signal waveform, is the essence of the present invention.